1. Field of the Invention
The present invention relates to a voltage-fed inverter device for use in speed control of an alternating current synchronous motor.
2. Description of the Related Art
FIG. 10 is a system arrangement diagram of a conventional inverter device, for example, disclosed at page 94 of “New Drive Electronics” edited by Naohiko Ueyama and published by Denki Shoin. This conventional inverter device includes a commercial alternating current power supply 1 for main circuit, a commercial alternating current power supply 2 for field system, a breaker 3, an input transformer 4, a main circuit 5 for power rectification using a semiconductor device (hereinafter referred to as power rectification main circuit), a smoothing capacitor 6, a main circuit 7 for inverse transformation using a semiconductor device (hereinafter referred to as inverse transformation main circuit), a breaker 8, an alternating current motor 9 driven by the inverter 7, and a resolver 10.
In the actual arrangement of the foregoing power rectification main circuit 5 and inverse transformation main circuit 7, as illustrated in FIG. 11, the power rectification main circuit 5 for converting alternating current power into direct current power is a circuit arranged by connecting series circuits, each composed of at least two electric switches connected in series with their conduction period controlled, in parallel by a predetermined number of phases, and a middle point of each series circuit is connected to an alternating current power supply S. On the other hand, the inverse transformation main circuit 7 for converting direct current power into alternating current power is a circuit arranged by connecting series circuits, each composed of at least two electric switches connected in series with their control period controlled, in parallel by a predetermined number of phases, and a middle point of each series circuit is connected to an alternating current load L.
The mentioned conventional inverter device also includes: an outside speed command generator 11; a contact 12 that acts to be turned on by an operation command signal from outside of an inverter circuit 25; a phase detector 13 that detects a phase on the basis of a signal outputted by the resolver 10; a speed detector 14 that computes a speed on the basis of a phase signal outputted by the phase detector 13; a speed controller 15 that compares a speed reference signal outputted by the outside speed command generator 11 with a speed feedback value outputted by the speed detector 14 and generates a current reference signal; an arithmetic circuit 16 that generates a main circuit current reference signal and a field current command signal on the basis of the current reference signal outputted by the speed controller 15 and the phase signal outputted by the phase detector 13; and a control circuit 18 that generates a current command value by comparing the main circuit current reference signal outputted by the arithmetic circuit 16 with a current feedback value of the inverse transformation main circuit 7 detected by a main circuit current sensor 17 and brings a gate signal of the semiconductor device of the inverse transformation main circuit 7 under PWM control according to the current command value.
The mentioned conventional inverter device further includes: a current sensor 19 that detects a power rectification main circuit current feedback value; a converter control circuit 20 that transmits and controls a gate signal of the semiconductor of the power rectification main circuit 5 using a direct current voltage command value preliminarily set in the mentioned current sensor 19 and an output of the mentioned current sensor 19; a field current sensor 21; a control circuit 22 that compares the field current command signal of the arithmetic circuit 16 with a field current feedback value detected by the current sensor 21 and controls a field current; and a field thyristor power supply 24. In addition, numeral 23 indicates an inverter control circuit, and numeral 25 indicates an inverter circuit for PWM control.
Operation of the mentioned conventional inverter system is now described. The arithmetic circuit 16 computes and obtains a main circuit current reference signal on the basis of a current reference signal of the speed controller 15 obtained by comparing and computing a speed reference signal from the outside speed command generator 11 with a speed feedback signal of the speed detector 14, as well as on the basis of a phase signal of the phase detector 13. The control circuit 18 compares the main circuit current reference signal outputted by the arithmetic circuit 16 with a main circuit alternating current feedback value of the inverse transformation main circuit 7 detected by the main circuit current sensor 17 and obtains a current command value. Then a gate signal under PWM control is added to the gate of the semiconductor of the inverse transformation main circuit 7 based on the current command value from the control circuit 18, thereby controlling strength of the current flowing through the alternating current motor 9. Therefore, a deviation value of the speed reference signal and the speed feedback signal represents the strength of the current flowing through the alternating current motor 9, and the flowing current becomes stronger as the deviation is larger.
As described above, in the system for controlling drive of the alternating current motor 9 by the inverter circuit 25, in a case where the alternating current motor 9 driven by the inverter circuit 25 is a synchronous motor, the alternating current motor 9 is driven with a voltage determined by a rated voltage value of the power semiconductor used in the inverse transformation main circuit 7 of the inverter circuit 25, and an alternating current output voltage generated by the inverse transformation main circuit 7 is lower than motor rated voltage in case of a motor driven with a commercial power supply.
Moreover, since the resolver 10 is used for the purpose of detecting a position of an armature of the alternating current motor 9, the resolver cannot resist high-speed rotation of the alternating current motor 9, and the conventional system is not applicable to a motor driven at a high speed.